Method and apparatus for providing high speed data communications in a cellular environment

ABSTRACT

A method and apparatus for transmitting digital data in a cellular environment. Adjacent cells of the cellular system are prevented from simultaneously transmitting data. Because the noise from transmissions of adjacent cells is a primary source of interference, the transmission rate of power limited base stations can be dramatically increased when the noise from adjacent cells is eliminated. The transmissions to each subscriber station are made at a fixed transmission power level. However, the data rate of transmitted signals differs from one subscriber station to another depending the path loss differences. In a first exemplary embodiment, the data rate of transmissions to a subscriber station is determined by selecting an encoding rate for the transmitted signal while holding the symbol rate constant. In a second exemplary embodiment, the data rate of transmissions to a subscriber station is determined by selection a modulation format for the transmitted signal which directly changes the symbol rate of transmission to a subscriber station.

CROSS REFERENCE

[0001] This application claims priority from U.S. application Ser. No.08/741,320, filed Oct. 26, 1996, entitled “Method and Apparatus forProviding High Speed Data Communications in a Cellular Environment” andcurrently assigned to the assignee of the present application.

BACKGROUND OF THE INVENTION

[0002] I. Field of the Invention

[0003] The present invention relates to communication systems. Moreparticularly, the present invention relates to a novel and improvedmethod and apparatus for providing high speed data in a wirelesscellular communication environment.

[0004] II. Description of the Related Art

[0005] As wireless communication technology has advanced, an increase inthe demand for high speed data services in a wireless environment hasgrown dramatically. The use of code division multiple access (CDMA)modulation is one of several techniques for providing digital wirelesstransmission that is well suited for the transmission of digital data.Other methods of digital wireless transmission include time divisionmultiple access (TDMA) and frequency division multiple access (FDMA).

[0006] However, the spread spectrum modulation technique of CDMA hassignificant advantages other digital modulation techniques. The use ofCDMA techniques in a multiple access communication system is disclosedin U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESSCOMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, assignedto the assignee of the present invention and incorporated by referenceherein. The use of CDMA techniques in a multiple access communicationsystem is further disclosed in U.S. Pat. No. 5,103,459, entitled “SYSTEMAND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONESYSTEM”, assigned to the assignee of the present invention andincorporated by reference herein. The method for providing digitalwireless communications using CDMA modulation was standardized by theTelecommunications Industry Association (TIA) in TIA/EIA/IS-95-A MobileStation-Base Station Compatibility Standard for Dual-Mode WidebandSpread Spectrum Cellular System (hereafter IS-95).

[0007] The current wireless communication systems can only accommodaterelatively low transmission rates. In addition, most current wirelesscommunication systems have not been optimized for the transmission ofdigital data, but rather have been optimized for the transmission ofspeech information. Therefore, there is a need in the industry for amethod of providing high speed digital data in a wireless environment.

SUMMARY OF THE INVENTION

[0008] The present invention is a novel and improved method andapparatus for transmitting digital data in a cellular environment. Inthe present invention, adjacent cells of the cellular system areprevented from simultaneously transmitting data. Thus, if a first basestation on one side of a cell boundary is transmitting, then a secondbase station on the other side of the cell boundary is silent throughoutthe transmission period of the first base station. Because the noisefrom transmissions of adjacent cells is a primary source ofinterference, the transmission rate of power limited base stations canbe dramatically increased when the noise from adjacent cells iseliminated.

[0009] In the present invention, all transmissions from a base stationare transmitted at a fixed power level and the transmissions to eachsubscriber station in a cell are transmitted in non overlapping bursts.Thus, when a base station is transmitting, its transmissions aredirected to one subscriber station within the cell, allowing the fullamount of available power to be used to transmit data to that subscriberstation which maximizes the available data rate to the subscriberstation.

[0010] For the sake of clarity, it should be noted that two separate butrelated rates are referred to herein. One is the information rate whichrefers to the rate of user generated information bits. The second is thetransmission rate which is the rate of bits transmitted over the air.

[0011] When transmissions are made at a fixed power level, the amount ofinformation that can be transmitted between the base station and thesubscriber station varies with link budget factors which are well knownin the art. The most significant link budget factor in a wirelesscommunication system is the path loss between the base station and thesubscriber station. The path loss is strong function of the distancebetween the base station and the subscriber station.

[0012] In the present invention, the transmissions to each subscriberstation are made at a fixed transmission power level. However, theinformation rate of transmitted signals differs depending the distancebetween the subscriber station and the base station. In the firstexemplary embodiment, the information rate of transmissions to asubscriber station is determined by selecting an encoding rate for thetransmitted signal while holding the transmission rate constant. In thesecond exemplary embodiment, the information rate of transmissions to asubscriber station is determined by selecting a modulation format forthe transmitted signal which directly changes the transmission rate oftransmission to a subscriber station.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The features, objects, and advantages of the present inventionwill become more apparent from the detailed description set forth belowwhen taken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

[0014]FIG. 1 is an illustration of a typical cell diagram for ageographical area;

[0015]FIG. 2 is an illustration of the interrelation of the base stationcontroller, the base stations and the subscriber stations;

[0016]FIG. 3 is an illustration of an exemplary timing diagram and frameformats of the present invention;

[0017]FIG. 4 is a block diagram illustrating a cell in the presentinvention;

[0018]FIG. 5 is a block diagram illustrating the base station of thepresent invention;

[0019]FIG. 6 is a block diagram illustrating the subscriber station ofthe present invention; and

[0020]FIG. 7 is an illustration of a cell divided into a large number ofnarrow sectors.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] In the following description, the same reference number is usedto describe both the cell or area serviced by a base station and thebase station itself. In the present invention, two adjacent cells areprohibited from simultaneously transmitting. Thus, in FIG. 1, when thebase station 1 is transmitting then the base stations 2A-2F areprevented from transmitting. The noise (N₀) experienced by a basestation transmitting in a cellular environment is described by equation(1) below:

N ₀ =N _(b) +N _(m) +N _(t) +N _(r),  (1)

[0022] where N_(b) is the noise from base stations in adjacent cells,N_(m) is the interference from multipath reflections and N_(t) is thethermal noise in the system and N_(r) accounts for all other sources ofnoise.

[0023] The noise value (N₀) limits the amount of information that can betransmitted in a power limited wireless communication system. Thepresent invention eliminates the noise from adjacent cells, N_(b), bypreventing any two adjacent cells from transmitting simultaneously. Inaddition, because a base station transmits to only one subscriberstation at a time, all of its available energy can be used for thetransmissions to that one subscriber station. Reducing the total noise(N₀) and increasing the power available for transmission to a givensubscriber station greatly increases available the information rate fortransmissions to the subscriber station.

[0024] Referring to FIG. 2, base station controller (BSC) 4 controls theoperation of a large number of base stations within a geographicalregion. In the present invention, BSC 4 coordinates the transmission bybase stations 1, 2A-2F and 3A-3L such that no two adjacent cells aresimultaneously transmitting. In the present invention, BSC 4 sends asignal to a selected one of base stations 1, 2A-2F and 3A-3L, directingthe selected base station to transmit for a predetermined time interval.

[0025] In a preferred implementation, the cells are grouped into sets ofnon adjacent cells wherein any of the cells within that set maysimultaneously transmit. For example, a first set of non adjacent cellsmay consist of cells 2A, 2C, 2E, 3C, 3K and 3G. A second set of nonadjacent cells may consist of cells 2B, 2D, 2F, 3A, 3E and 3I. In thispreferred implementation, BSC 4 selects the subset of non adjacent cellswhich can transmit and any or all cells within that set of non adjacentcells can transmit during that frame cycle.

[0026] Referring to the timing diagram of FIG. 3, BSC 4 sends a transmitmessage to base station 1 at time 0. In the preferred implementation,BSC 4 sends a message to all base stations of the set of nonadjacentbase stations which includes base station 1. In response to thatmessage, base station 1 transmits during the time interval from 0 to T.At time T, BSC 4 sends a transmit message to base station 2A directingbase station 2A to transmit during the time interval between time T andtime 2T. This process is repeated for each base station of base stations2B-2F as shown in FIG. 3. At time 7T, BSC 4 sends a message to basestation 1 which transmits during the time interval between time 7T and8T.

[0027] Note that when one of base stations 2A-2F are transmitting, it ispossible for a subset of base stations 2A-2F to be transmitting, so longas no two base stations share a common cell boundary. For example, whenbase station 2A is transmitting then cells 1, 2B, 3F, 3E, 3D and 2Fcannot transmit because they are adjacent to cell 2A. However, cells2C-2F may transmit during this period because they are not adjacent tocell 2A. In a preferred embodiment, the time intervals for transmissionare the same so as to reduce the management complexity of coordinatingthe transmissions of base stations in the system. It should be notedthat the use of varying time intervals is foreseen as a possibility.

[0028] In the exemplary embodiment, illustrated in FIG. 3, thetransmission cycle of cells follows a simple deterministic pattern. Itis understood that in simple deterministic transmission cycle, it is notnecessary for the base station to operate under the control of BSC 4because each base station can transmit at predetermined times withoutcontrol from BSC 4. In a preferred embodiment, the transmission cycle isnot determined by a simple deterministic pattern such as the oneillustrated in FIG. 3.

[0029] In the preferred embodiment, BSC 4 selects a base station or setof nonadjacent base stations which is to transmit in accordance with theamount of information queued for transmission in the base station or setof non adjacent base stations. In the preferred embodiment, BSC 4monitors the amount of messages that are in a queue maintained by eachbase station or set of non adjacent base stations and selects the basestation to transmit based on the amount of data in the queues.

[0030] Within each cell may be a plurality of subscriber stations, eachwhich require data to be transmitted to them by the base station servingthat cell. In the exemplary embodiment, the base station designates theidentity of the subscriber station to which it is transmitting by meansof a header. Referring to FIG. 3, in the first time interval (time 0 toT), base station 1 transmits to a selected subscriber station. In theexemplary embodiment, each frame is 2 ms in duration. The transmitteddata is provided with a header that identifies the selected subscriberstation.

[0031] In an alternative implementation, each cell is divided intonarrow sectors wherein each sector can be transmitted to independentlyof transmitting to any other sector in the cell. This can beaccomplished by means of highly directional antennas the design of whichis well known in the art. FIG. 7 illustrates a cell 600 served by basestation 510, which is divided into sectors 500A-500O. In thisembodiment, each cell of the communication system which is similarlysectorized transmits to a random sector or subset of sectors in it. Theprobability of overlapping simultaneous transmissions from adjacentsectors is small as long as each cell is divided into a sufficientlylarge number of sectors.

[0032] It should be noted, with reference to FIG. 3, that all forwardlink transmissions are provided at the same energy E₀, which wouldtypically be the maximum transmission energy allowed for by governmentregulations. Equation (2) below illustrates a general link budgetanalysis which describes the interrelation of parameters in a wirelesscommunication system with fixed power (E₀):

E ₀ =R(bits/s)(dB)+(Eb/No)_(req)(dB)+L _(s)(dB)+Lo(dB),  (2)

[0033] where E₀ is the fixed transmission energy of the base station, Ris the transmission rate, (Eb/No)req is the required signal to noiseratio for a given error rate, L_(s) is path loss in decibels and L_(o)is the other loses in decibels. The path loss, L_(s), depends stronglyon the distance between the base station and the subscriber station. Inthe present invention, either the transmission rate, R, or the requiredsignal to noise ratio, (Eb/No)_(req), is varied based on the distancebetween the subscriber station and the base station.

[0034] Referring to FIG. 4, three subscriber stations 6A, 6B and 6C arewithin the cell boundary 10 and as such are served by base station 1.The distances to the subscriber stations 6A, 6B and 6C are r1, r2 andr3, respectively. In an alternative embodiment, an effective distancecan be used wherein the effective distance is a metric which is selectedin accordance with the path loss between base station 1 and thereceiving subscriber station. It will be understood by one of skill inthe art that the effective distance is related to but not the same asthe physical distance between the base station and the subscriberstation. The effective distance is a function both of the physicaldistance and the course of the propagation path.

[0035] Referring back to equation (2), it can be seen that the effectsof differences in the path loss (L_(s)) can be offset holding all elseconstant by changing the value of (Eb/No)_(req). The value (Eb/No)_(req)depends on the error detection and correction techniques employed toprotect the transmitted data. The encoding rate refers to the ratio ofthe number of binary symbols output by the encoder to the number of bitsinput into the encoder. In general the higher the encoding rate of thetransmission system the greater the protection to the transmitted dataand the lower the required signal to noise ratio of the signal(Eb/No)_(req). Thus, in a first exemplary embodiment of the presentinvention, the encoding rate for transmissions to subscriber stations isselected based on the distance between the subscriber station and thebase station. Because communication systems are bandwidth limited, thehigher encoding rate employed results in lower data throughput of thesystem.

[0036] In equation (2), it can be seen that the effects of differencesin the path loss (L_(s)) can, also, be offset by changing the value ofthe transmission rate, R. The transmission rate, R, is given by theequation:

R=R _(s)·log₂ M,  (3)

[0037] where R_(s) is the number of symbols transmitted and M is thenumber of symbols in the modulation constellation. Thus, if the distancebetween the base station and the subscriber station is great, thetransmission rate, R, is reduced. In the present invention, thetransmission rate is varied by changing the modulation format to onewith more or less symbols in the modulation constellation. Whereas, whenthe distance between the base station and the subscriber station issmall, the transmission rate, R, is increased. In the second exemplaryembodiment, the symbol rate is set by selection of a modulation format.The information rate is the rate at which actual bits of uncoded userinformation is transmitted.

[0038] Assuming that the physical distance and the effective distancesto be closely related, base station 1 will transmit at a lowerinformation rate to subscriber station 6A than it will to subscriberstation 6B, since the effective distance to subscriber station 6A islonger than the effective distance to subscriber station 6B.

[0039] In the exemplary embodiment, each subscriber station transmits amessage indicating its location to the base station serving the cell inwhich it is located. In an alternative embodiment, methods ofpositioning which are well known in the art can be used by thecommunication station to estimate the location of the subscriberstation. In an alternative embodiment, the base station uses aneffective distance which is determined in accordance with a measurementof the path loss between the base station and the subscriber station.The measurement of path loss can be performed by transmitting a signalof a known power from the base station and measuring the received powerat the subscriber station. Similarly, the measurement of path loss canbe performed by transmitting a signal of a known power from thesubscriber station and measuring the received power at the base station.It should be noted that the references to distance between the basestation and the subscriber station apply equally to the physicaldistance and the effective distance based on measured path loss.

[0040] In the present invention the initial encoding rate or modulationformat are selected and provided initially during the service set upprocedure. Then the distance is tracked. If a sufficient change in thedistance results during the service a new encoding rate or modulationformat is selected in accordance with the new distance.

[0041] In the first exemplary embodiment, the base station selects anencoding rate in accordance with the distance between the base stationand the subscriber station. The base station transmits an indication ofthe selected encoding rate to the receiving subscriber station. Thereceiving subscriber station, in accordance with the selected encodingrate, selects a decoding format appropriate for use with the selectedencoding rate.

[0042] In the second exemplary embodiment, the base station selects amodulation format based on the distance between the base station and thesubscriber station. The base station then transmits an indication of theselected modulation format to the receiving subscriber station. Thereceiving subscriber station, in accordance with the selected modulationformat, sets up the demodulator appropriate for reception of the signalmodulated in accordance with the selected modulation format.

[0043] A block diagram of the exemplary embodiment of base station 1 isillustrated in FIG. 5. A block diagram of the exemplary embodiment ofsubscriber station 6A is illustrated in FIG. 6.

[0044] In the first exemplary embodiment, the encoding rate fortransmissions to a subscriber station is selected in accordance with thedistance between the base station and the subscriber station. Thus, theinformation rate is varied with the transmission rate, R, held fixed byselecting one of a plurality of encoding rates. First, subscriberstation 6A registers with base station 1. In the registration process,mobile station 6A alerts base station 1 of its existence and performsbasic system set up tasks as is well known in the art. An exemplaryembodiment for device registration is described in detail in U.S. Pat.No. 5,289,527, entitled “MOBILE COMMUNICATION DEVICE REGISTRATIONMETHOD” which is assigned to the assignee of the present invention andincorporated by reference herein.

[0045] In the exemplary embodiment, signal generator 218 of subscriberstation 6A generates a message indicating its location and provides themessage to transmission subsystem 216. Transmission subsystem 216encodes, modulates, upconverts and amplifies the message and providesthe message through duplexer 201 for transmission through antenna 200.The location message is received by antenna 120 and provided to receiversubsystem 118. Recevier subsystem 118 amplifies, downconverts,demodulates and decodes the received location message and provides it totransmission controller 104.

[0046] In the exemplary embodiment of the present invention, the mobilestation 6A transmits a message indicating its location to base station 1during the registration process. In addition, in the exemplaryembodiment, subscriber station 6A tracks its own movement and if thedistance changes by at least a certain amount, subscriber station 6Atransmits an indication of its new location. As described abovealternative methods for determining the subscriber station's location ormethods based upon the measured the path loss can be employed. In theexemplary embodiment, the location information is provided totransmission controller 104 of base station 1, which computes thedistance between base station 1 and subscriber station 6A.

[0047] Transmission controller 104 selects an encoding rate inaccordance with the distance between subscriber station 6A and basestation 1. In a preferred embodiment the distances between base station1 and subscriber station 6A is quantized in to discrete values asillustrated in FIG. 4. Referring to FIG. 4, all subscriber stations thatare located between base station 1 and the circle 7A would receiveinformation at a first encoding rate. All subscriber stations that arelocated between circle 7A and the circle 7B would receive information ata second encoding rate. All subscriber stations that are located betweencircle 7B and the circle 7C would receive information at a thirdencoding rate. For example, referring to FIG. 4, base station 1 may usea rate ½ code when transmitting to subscriber station 6B which is closeto base station 1. However, base station 1 may use a rate ⅛ code whentransmitting to subscriber station 6A which is far from base station 1.

[0048] If the distance between the base station and the subscriberstation is great, a higher encoding rate code will be selected. Whereas,when the distance between the base station and the subscriber station issmall, a lower encoding rate will be selected. Error correction anddetection methods employed at subscriber station 6A will permit a lowerrequired signal to noise ratio, (Eb/N0)_(req), for a given error rate.The lower the rate of coding, the greater the number of errors that canbe corrected and the lower the required signal to noise ratio(Eb/N0)_(req).

[0049] In the first exemplary embodiment, transmission controller 104selects the encoding rate as described above and sends an indication ofthe selected rate to subscriber station 6A. In the exemplary embodiment,the message indicating the encoding rate is transmitted over a pagingchannel during the registration process. Paging channels are used inwireless communication systems for sending short messages from a basestation to a subscriber station. In a preferred embodiment, thecommunication system permits base station 1 to change the encoding rateby subsequent messages transmitted on the traffic channel. One reason toprovide for changing the encoding rate is to allow for changes in thelocation of subscriber station 6A.

[0050] In the exemplary embodiment, the message indicating the selectedencoding rate is provided by transmission controller 104 to encoder 106which encodes the message. The encoded symbols from encoder 106 areprovided to interleaver 108, which reorders the symbols in accordancewith a predetermined reordering format. In the exemplary embodiment, theinterleaved symbols are provided to scrambler 110 which scrambles theinterleaved signal in accordance with a CDMA spreading format asdescribed in the aforementioned U.S. Pat. Nos. 4,901,307 and 5,103,459.

[0051] The scrambled read signal is provided to modulator 112 whichmodulates the signal in accordance with a predetermined modulationformat. In the exemplary embodiment, the modulation format for thepaging channel is quadrature phase shift keyed (QPSK) modulation. Themodulated signal is provided to transmitter 114, where it is upconvertedand amplified and transmitted through antenna 116.

[0052] The transmitted message indicating the encoding rate is receivedby antenna 200 and provided to receiver (RCVR) 202. Receiver 202downconverts and amplifies the received signal and provides the receivedsignal to demodulator 204. Demodulator 204 demodulates the receivedsignal. In the exemplary embodiment, demodulation format for the pagingchannel is a QPSK demodulation format. In the exemplary embodiment, thedemodulated signal is provided to equalizer 205. Equalizer 205 is achannel equalizer which reduces the effects of the propagationenvironment such as multipath effects. Channel equalizers are well knownin the art. The design and implementation of a channel equalizer isdisclosed in copending U.S. Pat. No. 5,692,006 entitled “AdaptiveDespreader”, filed Jul. 31, 1995, which is assigned to the assignee ofthe present invention and incorporated by reference herein.

[0053] The equalized signal is provided to descrambler 206 whichdescrambles the signal in accordance with a CDMA despreading formatdescribed in detail in the aforementioned U.S. Pat. Nos. 4,901,307 and5,103,459. The despread symbols are provided to de-interleaver 208 andreordered according to a predetermined de-interleaving format. Thereordered symbols are provided to decoder 210 which decodes the messageindicating the selected encoding rate and provides the decoded messageto control processor 212.

[0054] In response to the decoded message, control processor 212provides a signal to decoder 210 indicating a decoding format that willbe used for high speed data transmissions. In the exemplary embodiment,decoder 210 is capable of decoding a received signal in accordance witha plurality of trellis decoding formats where each decoding formatcorresponds to a corresponding different encoding format.

[0055] Referring back to FIG. 5, data to be transmitted to thesubscriber stations in cell 1 (subscriber stations 6A, 6B and 6C) isprovided to queue 100. The data is stored in queue 100 according to thesubscriber station to which it is to be transmitted. The data forsubscriber station 6A is stored in memory 102A, the data for subscriberstation 6B is stored in memory 102B, the data for subscriber station 6Cis stored in memory 102C, and so on. The different memory elements(102A-102N) are purely for illustrative purposes, it will be understoodthat the queue typically consists of a single memory device and theseparate memory devices illustrated simply refer to memory locationswithin the device.

[0056] At the first time interval (t=0), in FIG. 3, BSC 4 sends amessage to transmission controller 104 directing base station 1 totransmit. In response transmission controller 104 selects a receivingsubscriber station within its coverage area and the period of time thedata has been sitting in the queue. In a preferred embodiment, theselection of the receiving subscriber station is based on the amount ofdata queued for transmission to the subscriber stations in the coveragearea. Transmission controller 104 selectively provides a signal to oneof memory elements 102A-102N based on its selection of the receivingsubscriber station. In addition, in accordance with the receivingsubscriber station selected, transmission controller 104 provides asignal to encoder 106 indicating the encoding rate to be used fortransmissions to the selected subscriber station.

[0057] Transmission controller 104 provides, to encoder 106, a headermessage identifying the receiving subscriber station. In an exemplaryembodiment, encoder 106 encodes the header message using an encodingformat to be used to encode the headers for transmissions to allsubscriber stations. In an exemplary embodiment, the header informationis encoded separately from the rest of the data, so that a subscriberstation need not decode the very large amount of data transmitted duringthe transmission interval if it is not intended for that subscriberstation.

[0058] Transmission controller 104, then, provides a signal to memoryelement 102A directing it to provide data and specifying the maximumamount of data that can be transmitted to receiving subscriber station6A during the predetermined time interval. The predetermined maximum isthe maximum of information that can be transmitted to subscriber station6A within the time interval, T, at the selected encoding rate (R_(enc)),for the fixed transmission rate, R, as shown in equation (4) below.

Max Data=(R·T)/Renc  (4)

[0059] In response to the signal from transmission controller 104,memory element 102A provides an amount of data less than or equal to MaxData to encoder 106.

[0060] Encoder 106 encodes the data using the selected encoding formatand combines the encoded symbols of the header message with the encodedsymbols of data. In the exemplary embodiment, encoder 106 is capable ofencoding the data at a plurality of convolutional encoding rates. Forexample encoder 106 may be capable of encoding the data using a rate ½,⅓, ¼ and ⅕ convolutional encoding formats. Encoding rates can be variedto essentially any rate by using a combination of encoders typicallyused and data puncturing. Encoder 106 provides the encoded symbols tointerleaver 108.

[0061] Interleaver 108 reorders the symbols in accordance with apredetermined reordering format and provides the reordered symbols toscrambler 110. Scrambler 110 scrambles the symbols in accordance with apredetermined CDMA spreading format and provides the spread symbols tomodulator 112. It should be noted that because only one subscriberstation 6A is being transmitted to, the use of scrambler 110 is for thepurposes of scrambling the data for security purposes and to increasethe signal's immunity to narrow band noise and not for the purpose ofmultiple access communications.

[0062] Modulator 112 modulates the spread symbols in accordance with apredetermined modulation format. In the exemplary embodiment, modulator112 is a 16-ary QAM modulator. Modulator 112 provides the modulatedsymbols to transmitter (TMTR) 114. Transmitter 114 upconverts andamplifies the signal and transmits the signal through antenna 116.

[0063] The transmitted signal is received by subscriber station 6A atantenna 200. The received signal is provided to receiver (RCVR) 202.Receiver 202 downconverts and amplifies the received signal. Thereceived signal is provided to demodulator 204 which demodulates thesignal in accordance with a predetermined demodulation format. Thedemodulated signal is provided to equalizer 205 which is a channelequalizer as described above. The channel equalized signal is providedto descrambler 206 which descrambles the signal in accordance with apredetermined CDMA despreading format as described above. De-interleaver208 reorders the despread symbols and provides them to decoder 210.

[0064] In the exemplary embodiment, decoder 210 first decodes the headermessage contained in the reordered symbols. The header message isprovided to header check means 214 which verifies that the informationbeing transmitted is intended for subscriber station 6A. If the data isintended for subscriber station 6A, then the rest of the data isdecoded. When the header indicates the data is intended for the user ofsubscriber station 6A, header check 214 sends a signal to decoder 210indicating that the remaining information should be decoded. In analternative embodiment, all information is decoded and then the headeris checked after the decoding process.

[0065] Decoder 210 decodes the symbols in accordance with the selecteddecoding format from control processor 212. In the exemplary embodiment,decoder 210 decodes the reordered symbols in accordance with one of aplurality of trellis decoding formats selected based on the selectedencoding rate. The decoded symbols are then provided to the user ofsubscriber station 6A.

[0066] In the second exemplary embodiment, transmission controller 104selects the modulation format in accordance with the distance betweenthe base station and the mobile station. Base station 1 sends anindication of the selected modulation format to the subscriber station.The modulation format directly effects the transmission rate R.Referring to equation (2), all parameters are fixed in this case exceptthe path loss, Ls, and the transmission rate, R. Higher transmissionrates (R) are transmitted using a modulation format that contains alarger set of modulation symbols. For example, 28-ary quadratureamplitude modulation (QAM) can be used for transmission to subscriberstation near the base station. Whereas 16-ary QAM modulation would beused for transmission to subscriber stations further from the basestation.

[0067] In the exemplary embodiment, subscriber station 6A transmits amessage indicating its location to base station 1. In response, basestation 1 selects a modulation format. As described with respect to theprevious embodiment, the distances computed by transmission controller104 are quantized. The modulation format is selected in accordance withthe quantized distances. Referring to FIG. 4, all subscriber stationsthat are located between base station 1 and the circle 7A would receiveinformation using a first modulation format. All subscriber stationsthat are located between circle 7A and the circle 7B would receiveinformation using a second modulation format. All subscriber stationsthat are located between circle 7B and the circle 7C would receiveinformation at using a third modulation format. For example, referringto FIG. 4, base station 1 may use a QPSK modulation format whentransmitting to subscriber station 6B which is close to base station 1.By contrast, base station 1 may use a 64-ary Quadrature AmplitudeModulation (QAM) when transmitting to subscriber station 6A which is farfrom base station 1. In the exemplary embodiment, the message indicatingthe selected modulation format is transmitted over a paging channelduring the registration process. Again, in a preferred embodiment, thecommunication system permits base station 1 to change the modulationformat by subsequent messages transmitted on the paging channel.

[0068] The transmitted signal indicating the modulation format isreceived by subscriber station 6A as described above and provided tocontrol processor 212. Control processor 212 provides a signal todemodulator 204 indicating a demodulation format that will be used.Demodulator 204, of the second exemplary embodiment, is capable ofdemodulating a received signal in accordance with a plurality ofdemodulation formats. In response to the signal from control processor212, an appropriate demodulation format is selected.

[0069] Referring back to FIG. 5, data to be transmitted to thesubscriber stations in cell 1 (subscriber stations 6A, 6B and 6C) isprovided to queue 100. At the first time interval (t=0), BSC 4 sends amessage to transmission controller 104 directing base station 1 totransmit. In response to the signal, transmission controller 104 selectsa receiving subscriber station as described above. Transmissioncontroller 104 selectively provides a signal to one of memory elements102A-102N based on its selection of the subscriber station. In addition,in accordance with the subscriber station selected, transmissioncontroller 104 provides a signal indicating the selected modulationformat to modulator 112.

[0070] Transmission controller 104 provides, to encoder 106, a headermessage that identifies the subscriber station to which the data isbeing sent. Encoder 106 encodes the header message as described above.Transmission controller 104, then, provides a signal to memory element102A directing it to provide data and specifying the maximum amount ofdata that can be transmitted to receiving subscriber station 6A duringthe predetermined time interval. The predetermined maximum is themaximum of information that can be transmitted to subscriber station 6Awithin the time interval, T, at the selected rate as shown in equation(4) below.

Max Data=M·R _(s) ·T,  (5)

[0071] where M is the number of modulation symbols used in the selectedmodulation format and R_(s) is the symbol rate. In response to thesignal from transmission controller 104, memory element 102A provides anamount of data less than or equal to Max Data to encoder 106.

[0072] In the second exemplary embodiment, encoder 106 encodes the dataat a fixed encoding rate and combines the encoded symbols of the headermessage with the encoded symbols of data. Encoder 106 provides theencoded symbols to interleaver 108. Interleaver 108 reorders the symbolsin accordance with a predetermined reordering format and provides thereordered symbols to scrambler 110. Scrambler 110 scrambles the symbolsin accordance with a predetermined CDMA spreading format and providesthe scrambled symbols to modulator 112.

[0073] Modulator 112 modulates the scrambled symbols in accordance withthe selected modulation format. In the exemplary embodiment modulator112 is capable of mapping the scrambled symbols into modulation symbolsaccording to a plurality of modulation formats. Modulator 112 providesthe modulated symbols to transmitter (TMTR) 114. Transmitter 114upconverts and amplifies the signal and transmits the signal throughantenna 116.

[0074] The transmitted signal is received by subscriber station 6A atantenna 200. The received signal is provided to receiver (RCVR) 202.Receiver 202 downconverts and amplifies the received signal. Thereceived signal is provided to demodulator 204 which demodulates thesignal in accordance with the selected demodulation format. Thedemodulated signal is provided to equalizer 205 which channel equalizesthe received signal as described above. The equalized sign is providedto descrambler 206 which descrambles the signal in accordance with apredetermined CDMA despreading format. De-interleaver 208 reorders thedescrambled symbols and provides them to decoder 210.

[0075] In the exemplary embodiment, decoder 210 first decodes the headermessage contained in the reordered symbols. The header message isprovided to header check means 214 which verifies that the informationbeing transmitted is intended for subscriber station 6A. If the data isintended for subscriber station 6A then the rest of the data is decoded.When the header indicates the data is intended for the user ofsubscriber station 6A, header check 214 sends a signal to decoder 210indicating that the remaining information should be decoded. In analternative embodiment, all information is decoded and then the headeris checked after the decoding process is completed. Decoder 210 decodesthe symbols. The decoded symbols are then provided to the user ofsubscriber station 6A.

[0076] It should be noted that systems that use both varying theencoding rate and using the technique of varying the modulation formatsimultaneously are envisioned.

[0077] The previous description of the preferred embodiments is providedto enable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

I CLAIM:
 1. A method for a communication system, comprising: receiving asignal, carrying digital data, at a mobile station of a plurality ofmobile stations, from a base station, in non-overlapping transmissionbursts at a fixed predetermined power level, at a selected encodingrate, for a selected amount of data and with a selected modulationformat, wherein said non-overlapping transmission bursts are overpredefined and fixed duration time frames; decoding said received signalto retrieve said digital data based on said selected encoding rate, saidselected amount of data and said selected modulation format, wherein atleast one of said selected encoding rate, said selected modulationformat and said selected amount of data for said receiving at saidmobile station of said plurality of mobile stations from said basestation is based on an effective link budget for each of said pluralityof mobile stations, wherein said link budget includes a parameterindicating transmission set at said fixed and predetermined power level.2. The method as recited in claim 1 wherein said effective link budgetfor each of said plurality of mobile stations is based on an effectivedistance as measured by an effective transmission path loss between saidbase station and each of said plurality of mobile stations.
 3. Anapparatus for a communication system, comprising: a receiver forreceiving a signal, carrying digital data, at a mobile station of aplurality of mobile stations, from a base station, in non-overlappingtransmission bursts at a fixed predetermined power level, at a selectedencoding rate, for a selected amount of data and with a selectedmodulation format, wherein said non-overlapping transmission bursts areover predefined and fixed duration time frames; a decoder for decodingsaid received signal to retrieve said digital data based on saidselected encoding rate, said selected amount of data and said selectedmodulation format, wherein at least one of said selected encoding rate,said selected modulation format and said selected amount of data forsaid receiving at said mobile station of said plurality of mobilestations from said base station is based on an effective link budget foreach of said plurality of mobile stations, wherein said link budgetincludes a parameter indicating transmission set at said fixed andpredetermined power level.
 4. The apparatus as recited in claim 3wherein said effective link budget for each of said plurality of mobilestations is based on an effective distance as measured by an effectivetransmission path loss between said base station and each of saidplurality of mobile stations.
 5. A method for a communication system,comprising: selecting an encoding rate for each of fixed andpredetermined power level transmissions of digital data from a basestation to each mobile station of a plurality of mobile stations in saidcommunication system; selecting an amount of data for each of said fixedand predetermined power level transmissions from said base station toeach of said plurality of mobile stations; selecting a modulation formatfor each of said fixed and predetermined power level transmissions fromsaid base station to each of said plurality of mobile stations; encodingand modulating said selected amount of data in accordance with saidselected encoding data rate and said selected modulation format for eachof said plurality of mobile stations; transmitting from said basestation, encoded and modulated data, to each of said plurality of mobilestations in non-overlapping transmission bursts at said fixed andpredetermined power level, wherein said non-overlapping transmissionbursts are over predefined and fixed duration time frames, wherein atleast one of said selected encoding rate, said selected modulationformat and said selected amount of data for each of said fixed andpredetermined power level transmissions from said base station to eachof said plurality of mobile stations is based on an effective linkbudget for each of said plurality of mobile stations, wherein said linkbudget includes a parameter indicating transmission set at said fixedand predetermined power level; receiving and decoding said transmissionto retrieve said digital data based on said selected encoding rate, saidselected amount of data and said selected modulation format.
 6. Themethod as recited in claim 5 wherein said effective link budget for eachof said plurality of mobile stations is based on an effective distanceas measured by an effective transmission path loss between said basestation and each of said plurality of mobile stations.
 7. An apparatusfor a communication system, comprising: a controller for selecting anencoding rate, an amount of data and a modulation format for each offixed and predetermined power level transmissions of digital data from abase station to each mobile station of a plurality of mobile stations insaid communication system; a transmitter for encoding and modulatingsaid selected amount of data in accordance with said selected encodingdata rate and said selected modulation format for each of said pluralityof mobile stations, and for transmitting from said base station, encodedand modulated data, to each of said plurality of mobile stations innon-overlapping transmission bursts at said fixed and predeterminedpower level, wherein said non-overlapping transmission bursts are overpredefined and fixed duration time frames, wherein at least one of saidselected encoding rate, said selected modulation format and saidselected amount of data for each of said fixed and predetermined powerlevel transmissions from said base station to each of said plurality ofmobile stations is based on an effective link budget for each of saidplurality of mobile stations, wherein said link budget includes aparameter indicating transmission set at said fixed and predeterminedpower level; a receiver for receiving and decoding said transmission toretrieve said digital data based on said selected encoding rate, saidselected amount of data and said selected modulation format.
 8. Theapparatus as recited in claim 7 wherein said effective link budget foreach of said plurality of mobile stations is based on an effectivedistance as measured by an effective transmission path loss between saidbase station and each of said plurality of mobile stations.
 9. Anapparatus for a mobile station for communication with a base station ina communication system, comprising: a controller for selecting anencoding rate, an amount of data and a modulation format for a fixed andpredetermined power level transmission of digital data from said basestation; a transmitter for communicating said selected encoding rate,amount of data and modulation format to said base station, wherein saidfixed and predetermined power level transmission is over non-overlappingtransmission bursts from said base station, wherein said non-overlappingtransmission bursts are over predefined and fixed duration time frames,wherein at least one of said selected encoding rate, said selectedmodulation format and said selected amount of data for said fixed andpredetermined power level transmission is based on an effective linkbudget for said mobile station, wherein said link budget includes aparameter indicating transmission set at said fixed and predeterminedpower level.
 10. The apparatus as recited in claim 9 further comprising:a receiver for receiving and decoding said transmission from said basestation to retrieve said digital data based on said selected encodingrate, said selected amount of data and said selected modulation format.11. The apparatus as recited in claim 9, wherein said effective linkbudget for said mobile station is based on an effective distance asmeasured by an effective transmission path loss between said basestation and said mobile station.
 12. A method for a mobile station forcommunication with a base station in a communication system, comprising:selecting an encoding rate, an amount of data and a modulation formatfor a fixed and predetermined power level transmission of digital datafrom said base station; communicating said selected encoding rate,amount of data and modulation format to said base station, wherein saidfixed and predetermined power level transmission is over non-overlappingtransmission bursts from said base station, wherein said non-overlappingtransmission bursts are over predefined and fixed duration time frames,wherein at least one of said selected encoding rate, said selectedmodulation format and said selected amount of data for said fixed andpredetermined power level transmission is based on an effective linkbudget for said mobile station, wherein said link budget includes aparameter indicating transmission set at said fixed and predeterminedpower level.
 13. The method as recited in claim 12 further comprising:receiving and decoding said transmission from said base station toretrieve said digital data based on said selected encoding rate, saidselected amount of data and said selected modulation format.
 14. Themethod as recited in claim 12, wherein said effective link budget forsaid mobile station is based on an effective distance as measured by aneffective transmission path loss between said base station and saidmobile station.
 15. A method for a base station for communication with amobile station in a communication system, comprising: receivingcommunication from said mobile station including information relating toa selected encoding rate, an amount of data and a modulation format fora fixed and predetermined power level transmission of digital data fromsaid base station to said mobile station; encoding and modulating saidselected amount of data in accordance with said selected encoding datarate and said selected modulation format for transmission to said mobilestation, wherein said fixed and predetermined power level transmissionis over non-overlapping transmission bursts, wherein saidnon-overlapping transmission bursts are over predefined and fixedduration time frames, wherein at least one of said selected encodingrate, said selected modulation format and said selected amount of datafor said fixed and predetermined power level transmission is based on aneffective link budget for said mobile station, wherein said link budgetincludes a parameter indicating transmission set at said fixed andpredetermined power level.
 16. The method as recited in claim 15 furthercomprising: transmitting from said base station, encoded and modulateddata, to said mobile station in non-overlapping transmission bursts atsaid fixed and predetermined power level over said predefined and fixedduration time frames.
 17. The method as recited in claim 15, whereinsaid effective link budget for each of said mobile station is based onan effective distance as measured by an effective transmission path lossbetween said base station and said mobile station.
 18. An apparatus fora base station for communication with a mobile station in acommunication system, comprising: a receiver for receiving communicationfrom said mobile station including information relating to a selectedencoding rate, an amount of data and a modulation format for a fixed andpredetermined power level transmission of digital data from said basestation to said mobile station; an encoder for encoding and modulatingsaid selected amount of data in accordance with said selected encodingdata rate and said selected modulation format for transmission to saidmobile station, wherein said fixed and predetermined power leveltransmission is over non-overlapping transmission bursts, wherein saidnon-overlapping transmission bursts are over predefined and fixedduration time frames, wherein at least one of said selected encodingrate, said selected modulation format and said selected amount of datafor said fixed and predetermined power level transmission is based on aneffective link budget for said mobile station, wherein said link budgetincludes a parameter indicating transmission set at said fixed andpredetermined power level.
 19. The apparatus as recited in claim 18further comprising: a transmitter for transmitting from said basestation, encoded and modulated data, to said mobile station innon-overlapping transmission bursts at said fixed and predeterminedpower level over said predefined and fixed duration time frames.
 20. Theapparatus as recited in claim 18, wherein said effective link budget foreach of said mobile station is based on an effective distance asmeasured by an effective transmission path loss between said basestation and said mobile station.